1. Field of the Invention
The invention relates to a smectic, in particular a ferroelectric, liquid-crystal mixture based on phenylpyrimidine derivatives.
2. Description of the Prior Art
Liquid crystals (LCs), because of the unusual combination of anisotropic and fluid behavior, have a large number of possible uses in electrooptical switching and display devices.
In addition to nematic liquid-crystal phases, which have been employed for a long time, in recent times there has been increased use of smectic liquid-crystal phases too, in particular those which are ferroelectric (FLCs).
The practical use of ferroelectric liquid crystals in electrooptical switching and display elements requires chiral, tilted smectic phases such as S.sub.c * phases (see e.g. R. B. Meyer, L. Liebert, L. Strzelecki and P. Keller, J. Physique 36 (1975) L-69), which are stable over a broad temperature range. This aim can be achieved by using compounds which form such phases themselves or else by doping compounds which do not form chiral, tilted smectic phases with optically active compounds (see e.g. M. Brunet, Cl. Williams, Ann. Phys. 3 (1978) 237).
Switching and display devices containing ferroelectric liquid-crystal mixtures (FLC light valves) are disclosed, for example, in EP-A 0 032 362 (=U.S. Pat. No. 4,367,924). LC light valves are devices which modify their optical transmission properties, for example on the basis of electrical switchings, in such a way that light which is incident--and possibly reflected again--is intensity-modulated. Examples are the known watch and calculator displays or LC displays in the office communications and television sectors. However, they also include light shutters, as are employed, for example, in photocopiers and printers. So-called "spatial light modulators" are also areas of application for LC light valves (see Liquid Crystal Device Handbook, Nikkan Kogyo Shimbun, Tokyo, 1989; ISBN 4-526-02590-9C 3054 and papers cited therein).
The electrooptical switching and display devices under discussion are generally constructed in such a way that the FLC layer is enclosed on both sides by layers which are usually, in this sequence starting from the FLC layer at least one alignment layer, electrodes and a limiting plate (for example made of glass). In addition, they contain a polarizer if they are operated in "guest-host" mode or in reflective mode, or two polarizers if the transmissive birefringence mode is used. The switching and display elements may, if desired, contain further auxiliary layers, such as diffusion barrier or insulation layers.
The abovementioned alignment layers are conventionally rubbed films comprising organic polymers or obliquely vapor-deposited silicon oxide, and vary from display manufacturer to display manufacturer.
When the distance between the limiting plates is sufficiently small, the alignment layers bring the FLC molecules into a configuration in which the molecules lie with their long axes parallel to one another and the smectic planes arranged perpendicular or inclined to the alignment layer. In this arrangement the molecules have two equivalent alignments between which they can be switched by applying an electrical field in a pulsed manner.
In order to achieve a uniform planar alignment in the S.sub.c * phase over the entire display, it is advantageous if the phase sequence of the liquid-crystal mixture with decreasing temperature is: isotropic-nematic-smectic A-smectic C (see e.g. K. Flatischler et al., Mol. Cryst. Liq. Cryst. 131 (1985) 21; T. Matsumoto et al., pp. 468-470, Proc. of the 6th Int. Display Research Conf. Japan Display, 30 Sep.-2 Oct. 1986, Tokyo, Japan; M. Murakami et al., ibid. pp. 344-347).
For ferroelectric (chiral smectic) liquid-crystal mixtures, an additional condition which must be fulfilled is that the pitch of the helix in the S.sub.c * phase is sufficiently large to prevent the formation of a helix in the display, and is sufficiently large in the N* phase that it is a homogeneous nematic phase and not a twisted state which forms in the display during the cooling process. The formation of a uniform planar alignment in the display is necessary to obtain a high contrast.
Switching the molecules back and forth (and thus the light or dark position in the case of a fixed polarizer setting) is carried out, as already mentioned, by applying an electrical field in a pulsed manner. Because of the bistability of the FLC molecules, voltage must only be applied for one change in alignment. A subdivision of the display into individual pixels is achieved by the known matrix arrangement of the electrodes. As a rule, the electrodes are located on the insides of the carrier plates of the display with the rows on one carrier plate and the columns on the other carrier plate. In the regions of intersection, the pixels B, the liquid crystal located between rows and columns is switched. A comprehensive description of multiplex addressing for FLC displays can be found, for example in Proc. SID 28/2 (1978) 211 and Ferroelectrics 94 (1989) 3. The response times T of the FLC mixture in the display are inversely proportional to the spontaneous polarization P.sub.s and are in the .mu.s range. ##EQU1## E=level of applied electrical field .eta.=rotational viscosity
Another important factor besides the spontaneous polarization is the tilt angle .theta., that is the angle between the n director, i.e. the average molecular orientation, and the normal to the layers. Together with the birefringence .DELTA.n and the layer thickness d, this angle affects the brightness of the display in accordance with the relationship: ##EQU2## where T.sub.o is the intensity and .lambda. is the wavelength of the incident light.
Conventionally, given a matrix arrangement of the electrodes in the display, the term columns is applied to those electrodes which are subjected to information-carrying pulses (also called column or data pulses). The rows are then activated by electrical pulses in a stroboscopic sequence, which is the precondition for the transmission of information to the pixels of the rows. An important property of the display is the time required for setting up or changing an image. For many applications it should be as short as possible.
Since the rows are at best sequentially addressed, the critical factor is the duration for which a row must be addressed in order to read in the information. The shorter the voltage pulse required for switching the liquid crystal, the shorter this read-in time is. In general, the maximum voltage to be applied is predetermined by the choice of drivers, so that the pulse width necessary for switching should be as small as possible.
The product of necessary pulse width and voltage (=pulse height) is in good approximation constant, and thus independent of the voltage, so that the precise pulse area required for switching (CPA=critical pulse area) represents a parameter which is a good characteristic of the rapidity of the liquid crystal. The CPA should be as small as possible.
Furthermore, it is advantageous if the LC mixture in the display has a high margin with low flicker (J. Dijon et al., Ferroelectrics 113 (1991) 371). The term margin is understood as the voltage range, with a given driving scheme, in which the pulse height must lie in order that the LC mixture switches fully. The margin is influenced by the so-called bias, i.e. the ratio between the row and data pulse voltage. The margin should be as large as possible in order to compensate fluctuations in thickness and/or temperature in the display. In multiplex addressing, the molecules of nonselected rows are diverted by the data pulses out of their position of rest, and then relax again. The fluctuation in brightness which this causes is termed flicker. Flicker leads to a reduction in contrast. Rieker et al. (Phys. Rev. Lett. 59 (1987) 2658) showed that, when cooling from the isotropic phase through the S.sub.A phase, the S.sub.c phase forms a so-called chevron geometry in displays, i.e. that the layers are angled. Therefore, the effective tilt angle .theta..sub.eff must be inserted in the abovementioned relationship between tilt angle and transmission. The effective tilt angle is the angle between the projections of smectic normals and the optical axis of the liquid crystal to the glass surface of the cell.
A display can be operated alternatively in the chevron geometry, which is adopted naturally during the cooling procedure, or in the so-called quasi-bookshelf geometry (QBG), into which the liquid crystal can be brought by specific field treatment (see e.g. H. Rieger et al., SID 91 Digest (Anaheim) 1991, p. 396).
Obtaining good values for the majority of the above-listed parameters is barely possible with individual substances. Consequently, the transition was made many years ago to using mixtures of different substances. Such mixtures generally comprise an achiral basic mixture and optically active dopants.
The achiral basic components are intended to achieve a broad S.sub.c phase situated in a favorable temperature range. In addition, the achiral basic mixture should have the phase sequence I-N-S.sub.A -S.sub.c and as low as possible a melting point. The optically, active dopants are then used to induce the ferroelectric nature of the mixture, for pitch compensation and for adaptation of the optical and dielectric anisotropy.
It is known that certain derivatives of phenylpyrimidine, especially 5-alkyl-2-(4-alkoxyphenyl)pyrimidines, are able to form S.sub.c, S.sub.A and N phases (D. Demus and H. Zaschke, "Flussigkristalle in Tabellen" [Liquid Crystals in Tables], VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig 1974, pp. 260-261) and, in addition, can be converted into FLC mixtures by the addition of optically active dopants (M. L. Blino v et al., Sov. Phys. Usp., 27(7) (1984) 492; L. A. Beresnev et al., Ferroelectrics 59 [321]/1(1984), presented at the 5th Conference of Soc. Countries in Liquid Crystals, Odessa, USSR, Oct. 1983; DE-A 35 15 347, EP-A 0 206 228, EP-A 0 225 195). It is also known that relatively low melting points and a broadening in the desired liquid-crystalline phases can be achieved by mixing a plurality of LC compounds [see e.g. D. Demus et al., Mol. Cryst. Liq. Cryst. 25 (1974) 215; J. W. Goodby, Ferroelectrics 49 (1985) 275], and that the depression of melting point is greater, the greater the structural differences between the components of the mixture (see e.g. J. S. Dave et al., J. Chem. Soc. 1955, 4305).
Despite the successes in the provision of new LC materials which have been achieved by previous mixtures, the development of LC basic mixtures, and especially FLC mixtures, can in no way be regarded as at an end. Furthermore, manufacturers of display elements (displays) are interested in a broad spectrum of various mixtures for the various areas of application.